The impacts of harvesting and the sustainability of a New Zealand Littleneck Clam (Austrovenus stutchburyi) fishery in Papanui and Waitati inlets, New Zealand

Irwin, Craig Robert

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Irwin, C. R. (2004). The impacts of harvesting and the sustainability of a New Zealand Littleneck Clam (Austrovenus stutchburyi) fishery in Papanui and Waitati inlets, New Zealand (Thesis, Doctor of Philosophy). University of Otago. Retrieved from http://hdl.handle.net/10523/3377

The New Zealand Littleneck Clam (Austrovenus stutchburyi) fishery in the Otago province of New Zealand is based in Papanui and Waitati Inlets. A single operator, Southern Clams Limited, has commercially harvested the resource for 20 years. The management of Southern Clams Limited believe that harvesting of A. stutchburyi will encourage faster growth and higher recruitment levels in the remaining shellfish populations. It is also contended that commercial harvesting at present levels has little effect on the overall biomass of each inlet. This work aims to assess if these assumptions are valid and to comment on possible options available to the fisheries management to ensure the resources sustainability.

Biomass surveys were carried out to estimate biomass in each inlet and to compare biomass to estimates from previous surveys. The immediate impacts of harvesting was assessed as were the mechanisms (growth, recruitment and migration) of recovery. The impact of harvesting on associated macroinfaunal communities and sediment structure was also measured. The viability of re-seeding adult stock as a management tool was assessed by re-seeding harvest areas at three density levels and monitoring growth of the moved clams and recruitment of new clams into the re-seeded areas. Cage experiments studied the importance of the origin of re-seeded clams in relation to growth and mortality rates.

Biomass has consistently decreased since commercial harvesting began. A large decrease in juvenile biomass was found in both inlets. Harvesting had direct and immediate impacts on biomass and density levels, length frequency distribution and the spatial patterns of the remaining population. Biomass and density showed little sign of recovery in three years. Harvesting had a severe but short lived impact on macroinfaunal community structure and no change in sediment structure was found after harvesting. Any increase in growth of re-seeded clams appeared to be offset by high levels of mortality. Clams from slow growth areas were able to increase their rate of growth when moved to more productive areas but were less able to cope with the stress of movement. Evidence was found that growth increased when adult density was low but doubt surrounds the assumption that recruitment will increase as adult density decreases.

At current harvest levels this fishery does not appear to be sustainable. Biomass removed by harvesting is not being replaced, especially in Papanui Inlet. Major reductions in juvenile biomass indicate that this resource may be subject to recruitment overfishing. Re-seeding appears to be a viable option if used to maintain community structure and stability after harvesting and to help reduce the uncertainty associated with recruitment. To ensure the sustainability of this resource it is suggested that high spawner stock biomass be protected and a rotational fishery plan implemented. In future years hatchery reared stock may become important to help restock harvested areas and guarantee a consistent supply of recruitment. Continued research and gathering of quality information on which to base management decisions is vital to the future sustainability of this fishery.